Steam inlet components for power generating turbines currently suffer from sliding and fretting wear damage during the operation of the turbine. Moreover, temperature cycles of more than 500.degree. F. and the resulting stress create an additional threat of failure of these parts due to thermally induced cracking.
Traditionally, steam inlet components for turbines have included nozzle chambers with pistons or bell seals disposed therein. These components operate in a harsh environment including exposure to high pressure steam, at conditions of about 1100.degree. F. and 3500 psi. Under these conditions, caution must be exercised in selecting materials which provide the necessary metallurgical, physical and mechanical property relationships. The material choices usually involve metals having dissimilar coefficients of expansion, oxidation resistance, corrosion resistance, and high temperature wear resistance. Ultimately, it is desirable to choose a combination of alloys that minimizes the wear of the inlet steam sealing surfaces, while at the same time, providing for satisfactory steam sealing properties.
Metallurgical analysis of selected mating components of the inlet steam chambers, such as, nozzle chamber walls, sealing rings and STELLITE bells, has revealed service induced abrasive and adhesive wear. These pressure retaining parts have also been known to undergo sufficient material loss, resulting in unacceptable steam leakage.
In typical floating piston systems, the nozzle chambers are manufactured using 2.25% Cr, 1% Mo steel (ASTM A182, Gr F22). These systems also include steam sealing rings which are typically made from "REFRACTORY 26" from Carpenter Technology. The pistons of such systems, on the other hand, are usually fabricated from 12% Cr, heat resistant stainless steel (AISI 616), Bethlehem Steel Corp. In conventional bell seal systems, the sealing surfaces include a STELLITE bell against a 21/4% Cr-1% Mo chamber wall. Although selected for their corrosion resistance and steam sealing capabilities, the materials of these systems have been less than optimum in their ability to resist wear and steam leakage during prolonged operation of the turbine.
Careful analysis of the movement of floating piston systems, at the location where the piston comes in contact with the steam inlet chamber of a turbine, has revealed various stages of wear damage. Initially, there is metal to metal friction caused by the rings sliding against the nozzle chamber. Since, at high temperatures, these rings are significantly harder than the nozzle chamber, the sliding results in abrasive wear damage of the nozzle i.e. 2.25% Cr - 1.0% Mo steel, which oxidize. These oxide particles are harder than the surfaces of the rings and the chamber, and accordingly, abrade both surfaces. As the wear mechanism progresses, there occurs a continuous oxidation of fresh surfaces.
In addition to this mechanism, adhesive wear also occurs at the contact point between the chamber and the rings of floating piston systems. Occasionally, during the operation of the steam turbine, two clean ring and chamber surfaces come into contact due to breakage of the protective oxide film in local areas of these components. When the applied stresses at these contact areas exceed a critical value, cold welding, or adhesive wear, can occur at the contact junction. If this condition is left to develop further, the resulting sheer stresses can cause the delamination of one of the materials at the contact surface. Since the 2.25% Cr - 1.0% Mo steel is relatively soft, the nozzle surface transfers metal to the relatively hard ring surfaces. Any tilt or twist in the rings or lack of a complete contact between a ring and the nozzle chamber can significantly accelerate the deterioration of the steam inlet components.
Similar wear related problems have also been found in the bell seal design, where the harder STELLITE bell sealing surface wears the 21/4% Cr-1% Mo interior wall of the steam chamber.
Replacement of worn, fretted, or cracked components can be extremely costly. Down-time alone can amount to hundreds of thousands of dollars per day, since an electric utility must buy electrical power elsewhere to meet consumer demands. In addition to this cost, the expenses associated with hiring a repair crew and purchasing and storing spare parts can be significant.
Accordingly, there is still a need for a material combination and steam chamber design that minimizes sliding and fretting wear of inlet steam sealing surfaces. There is also a need for extending the useful life of nozzle chambers to minimize down-time of the steam turbine.